Hopp til hovedteksten
Fjord
Print friendly version

Fjords - water exchange and currents

The exchange of water masses between fjords and the coast is a result of several dynamical components. The bottom depths and formation of land is also of course important for the circulation pattern.

It is reasonable to characterize the exchange of water masses within depth zones, such as a surface layer (0-5m depth), an intermediary layer (between the surface layer and the depth of the fjord sill) and a fjord basin (below the sill depth). The surface layer is mainly transporting less saline water offshore, from the fjord and into the Norwegian Coastal Current. A compensating current may arise below this surface layer. The currents within the intermediary layer are dependent on the density gradients between offshore and inshore waters, and the resulting transports are mostly much larger than within the surface layer. The fjord basins often consist of stagnant water masses, but they may occasionally be replaced with offshore coastal water.

vannmasseutvekslingen
Figure 1. Schematic illustration of water mass exchange between inshore and offshore coastal waters.

The fjords and inshore coastal waters receives a variable and amount of fresh water from rivers, and the surface layer is characterized with relatively low salinities. The river runoff has a clear seasonal signal, typically with maximum discharge during the spring related to melting of inland snow, and a minimum during the winter. The Norwegian west coast also has a typical maximum runoff during autumn due to enhanced precipitation rates. Since river runoff is a result of the local precipitation, inter-annual variations are often large. The effect of the earth’s rotation (Coriolis) on the circulation is important if the fjord has a certain width (at least 2-3 km). In these fjords, the currents will often turn to the right and have larger speed closer to land. In more narrow fjords, where the Coriolis force is less dominant, the circulation is more constant across the fjord, and the in- and outflow will take place in different depths.

Currents

The currents in fjords are strongest and most variable in the upper 10-20m. Besides topographical distinctions, the currents are driven by river runoffs, winds, tides and water exchange due to offshore density differences. Strong tidal currents are often found in narrow straits and above shallow sills, while periodical strong currents in the open parts of the fjords may form due to strong local winds. The wind driven currents are mainly prominent in the upper 10-20m and are strongest near the surface. The wind driven currents may constitute between 3 and 8% of the wind speed, and its impact on currents is most evident when there is a strong vertical stratification. Along with strong winds the currents may exceed 100 cm/s at the surface and 50 cm/s at 10m-depth. At normal conditions the currents are below 30 cm/s. In bays and fjord arms the currents may be much weaker than in the open fjord- and coastal areas. Only a few current observations in fjords exist historically, but recent efforts include numerical ocean models (e.g. NorKyst800) combined with current measurements for model validation to increase our knowledge of fjord and coastal dynamics. 

3-D numerisk fjordmodel
Figure 2. Average surface current speed and direction (upper panel) and standard deviation in speed (lower panel) for an area between Vikingnes and Varald Island in the Hardangerfjord estimated by a 3-D numerical fjord model with a horizontal resolution of 200m.

 

 

Fixed hydrographic stations

Measurements of temperature and salinity at 8 fixed hydrographic stations distributed along the Norwegian coast. The observations are taken 1-2 times per month.

Contact

Lars Asplin
994 04 871
Jon Albretsen
913 04 222
Anne Dagrun Sandvik
47 40 80 20